United States laws and regulations obligate a feed oil pipe for an automobile to assure life of 15 years or 150,000 miles running. A feed oil pipe made of stainless steel (SUS436L: 17 Cr-1.2Mo) has been already put into practical use.
An automobile traveling in North America or Europe is exposed to an environment of a snow melting salt. Therefore, a material applied to a feed oil pipe requires excellent salt corrosion resistance, and SUS436L has been conventionally applied. However, in response to the recent increase in resource prices, reduction in material cost has come to be demanded. SUS436L contains about 1% of expensive Mo. Therefore, only by replacing SUS436L with AISI439 steel (17 Cr) not containing Mo, cost is significantly reduced. In a district other than North America or Europe (e.g. India, China, or South and Central America), since salt corrosion resistance at such a level as in North America or Europe is not necessary, the quality of SUS436L is excessive and a lower-grade and more inexpensive material has been demanded.
However, when the amount of an alloy element is excessively reduced in order to reduce cost, corrosion resistance is deteriorated. Therefore, a technology to compensate weakness due to a lower-grade material by another method is important.
A portion having a concern about corrosion in a feed oil pipe is crevice corrosion which occurs in a gap on an outer surface of the feed oil pipe exposed to a chloride environment. Conventionally, coating such as cation electrodeposition coating has been applied as a means for improving salt corrosion resistance in the gap.
For example, Patent Literature 1 discloses a production method of subjecting a feed oil pipe assembled by projection welding using a SUS436 pipe as a material to cation electrodeposition coating. However, this technology uses SUS436 as a material. According to knowledge of the inventors, rust prevention is not completely achieved even in SUS436. Therefore, it cannot be estimated that this technology can offer a sufficient rust prevention effect when a lower-grade material is used.
Patent Literature 2 discloses a technology of preventing crevice corrosion by subjecting a feed oil pipe assembled using SUS436 as a material to electrostatic coating. Alternatively, Patent Literature 3 discloses a technology of subjecting a stainless steel feed oil pipe to coating for chipping resistance and assuring a sufficient rust prevention property even when the feed oil pipe is subjected to chipping. However, these technologies require higher cost for coating than electrodeposition coating. Meanwhile, an inside of a gap cannot be coated, and therefore there is no guarantee that a sufficient rust prevention effect is obtained in the gap.
Patent Literature 4 discloses a technology of providing a projection on a gap-forming member and controlling an opening amount of the gap to 0.2 mm or more for covering an inside of the gap by electrodeposition coating.
On the other hand, a rust prevention method other than coating is also proposed. For example, Patent Literature 5 discloses a technology of applying sacrificial corrosion prevention by disposing a zinc sacrificial anode in a gap or a portion in which a passive state film is impaired by welding, brazing, plastic working, or the like in assembling a stainless steel feed oil pipe. However, it is troublesome and takes time to dispose zinc at all the portions having a concern about corrosion. As described in Non Patent Literature 1, zinc is consumed easily in a chloride environment, and therefore a large amount of zinc is disadvantageously required. In Patent Literature 6, a galvanized steel plate is used for an inlet pipe to eliminate a gap by filling the gap with melted zinc. However, as described above, zinc is consumed very easily in a chloride environment. Further, since an oil inlet enters an inlet pipe, melted zinc is likely to invade the inlet pipe to be reacted with water, so that a corrosion product such as zinc hydroxide may be formed to cause clogging of fuel injection equipment.